The reeling mill (reeler) is layed between the plug mill and sizing mill in the process for manufacture of seamless steel pipes. In this mill, the pipe is "reeled" by reducing its wall thickness for various purposes among which are the correction of the thickness deviation that has developed in the previous rolling mills (i.e. piercer, elongator and plug mill), elimination of the flaws developed by abrasion with the plug in the plug mill rolling, and ensuring a suitable amount of pipe expansion for the sizing mill in the subsequent step. It is the most important feature of the reeling mill that the greater part of the amount of reduction in wall thickness changes into the expansion of the outside diameter of the pipe, so if a variation occurs in the amount of reduction of the wall thickness during reeling, the outside diameter of the reeled pipe also changes in its longitudinal direction and the following disadvantages result: if the amount of reduction in wall thickness during reeling is small, the degree of increase in the outside diameter is small and the desired outside diameter of the pipe cannot be achieved, and vice versa. If the outside diameter of the reeled pipe is smaller than a predetermined value, not all parts of the pipe is rolled in the subsequent sizing mill, which has a very adverse effect on the outside diameter of the final product. If the outside diameter of the reeled pipe is larger than the predetermined value, a flaw such as one due to the roll edges develops during rolling in the sizing mill, which is also detrimental to the quality of the final product. So, the operation in the reeling mill has great effect on the accuracy of the dimensions of the final product, hence its yield, and it is mandatory in the reeling mill to roll the pipe to have the desired outside diameter.
Several methods are known for controlling the operation of reeling mills. One of them is described in Japanese Patent Application (OPI) No. 37568/78 (the symbol OPI as used herein means an unexamined published Japanese Patent Application), and to provide a pipe of constant outside diameter on the leaving side of the mill, the electric power for rolling is kept constant for each lot and for the entire length of each pipe to be rolled. Another method is described in Japanese Patent Application (OPI) No. 86663/78, and to provide a pipe of constant wall thickness on the leaving side, the pattern of electrical power outputs is determined for each pipe on the basis of information of the cross section and temperature of the pipe on the entry side so that the cross section of the pipe on the leaving side is kept constant for each lot and for the entire length of each pipe, and the electric power for rolling is changed after this pattern.
The method of Japanese Patent Application (OPI) No. 37568/78 disregards the change in the temperature of each pipe to be rolled as well as the change in the temperature in the longitudinal direction of the pipe. So, if the reeling operation is performed with the electric power held constant, the reduction in the wall thickness at the low temperature portion of pipe where occurs the larger resistance to deformation is so small that the pipe on the leaving side cannot be expanded to the predetermined value of outside diameter. What is more, the outside diameter of the pipe on the entry side of the mill is also varied in longitudinal direction by the rolling force and by the set of roll gap in the plug mill rolling, so this adds to the variation in the outside diameter of the pipe that is leaving the reeling mill under the control of constant electric power.
The method of Japanese Patent Application (OPI) No. 86663/78 achieves its object by changing the desired electric power according to a predetermined pattern by taking into account the change in the temperature of the pipe in its longitudinal direction, but as in the first method, it disregards the change in the outside diameter of the pipe on the entry side in defining the target value for the electric power that provides a pipe of constant cross-sectional area on the leaving side of the mill. As a result, there occurs a variation in the outside diameter of the reeled pipe both in the longitudinal direction and with respect to the average outside diameter of the pipes for each lot. The second method has another defect: the pipe to be fed into the plug mill generally has such a temperature distribution in the longitudinal direction that the temperature increases from the leading end to the trailing end, and so the wall thickness of the pipe leaving the plug mill often increases from the leading end to the trailing end. Therefore, the pipe entering the reeling mill generally has a cross section that decreases from the loading end to the trailing end.
To reel the pipe having a cross-sectional area of the pattern described above and provide a product whose cross section is uniform in the longitudinal direction, the amount of reduction in wall thickness must be decreased from the leading to trailing edge, but then, the resulting pipe on the leaving side has an outside diameter that decreases from the leading to trailing end and which is not desired in the final product. Still another defect that is common to the two methods is that in spite of the supply of a constant electric power for rolling, a change in the speed of the rolling motor during reeling causes change in the rolling torque, and as a result, the desired outside diameter is not attained.
Therefore, one object of this invention is to provide a control method for screwing down the reeling rolls that is free from the above described defects of the conventional techniques and which keeps providing a pipe of constant outside diameter on the leaving side of the reeling mill by first determining the desired amount of reduction in wall thickness in the longitudinal direction of the pipe being reeled, calculating the actual amount of reduction in wall thickness from the torque of the rolling motor and the temperature of the pipe on the entry side and controlling said actual amount of reduction in wall thickness to be equal to the desired amount of reduction in wall thickness.
Another object of this invention is to provide a control method for screwing down the reeling rolls that achieves quantitative adjustment of the roll gap so that the pipe leaving the reeling mill has a uniform outside diameter in the longitudinal direction.
Still another object of this invention is to provide an apparatus that is used with advantage in performing the above described control methods.